1. Field of the Invention
The invention relates to a JFET current mirror of the type employed in the voltage level shifting section of an operational amplifier.
2. Description of the Related Art
There are a number of current mirrors known to those of ordinary skill in the art but the use of JFETS in the context of current mirror is virtually unknown. FIG. 1 discloses a prior art current mirror using transistors. Assuming that transistor T2 is not saturated, current I2 will set itself equal to current I1 since the base emitter voltage of transistor T1 is equal to the base emitter voltage of transitor T2.
FIG. 2 describes another prior art current mirror employing NMOS technology. Current I2 will again set itself equal to current I1 since the gate source voltage of transistor T3 is always equal to the gate source voltage of transistor T4. Due to second order effects I2 is only approximately equal to I1. An improved circuit, known as a Widlar current mirror sets I2 closer to I1 and is shown in FIG. 3 as prior art. While the foregoing circuits operate satisfactorily for the conventional semiconductors shown, they do not work with JFETS.
Techniques are known in the context of conventional semiconductor art for biasing active devices. For example, U.S. Pat. No. 3,953,087 discloses one technique for producing a voltage drop across a semiconductor element in the context of an amplifier. However, the nature of the amplifier and the purpose for achieving the voltage drop is significantly different from that of the present invention. Another technique for biasing a semiconductor is described in U.S. Pat. No. 4,460,864 in the context of a voltage reference circuit. Lastly, U.S. Pat. Nos. 4,158,178 and 4,217,540 describe other schemes for biasing semiconductors. Insofar as understood, none of the prior art teaches or suggests useful methods for biasing a JFET in the unique context of a current mirror.